a) Field of the invention
This invention relates to a high-accuracy position comparator for performing the measurement of a dimensional profile of an object and the positional comparison between two objects, with a high degree of accuracy, by using moire fringe gratings, laser scales, graduated rulers or the like.
b) Description of the prior art
In the past, instruments have been known in which the dimension of an object is measured with a considerable degree of accuracy by using moire fringe gratings, laser scales, graduated rulers or the like as standard scales and the positions of two objects are made to coincide accurately with each other.
Each of these instruments, for example, as shown in FIG. 1, comprises a base 1, a standard scale 2 fixed to the base 1, a stationary bed 3 protruding from the surface of the base 1, and a movable bed 4 slidable along the surface of the base 1 so that whenever the length of an object being measured 5 is determined, the object 5 is interposed between the stationary bed 3 and the movable bed 4 to read out the position of the movable bed 4 on the standard scale 2. If, however, an axis of measurement is inclined as depicted in FIG. 2, an error will be produced as to the reading-out of the standard scale 2. To prevent such a difficulty, the so-called Abbe's principle is applied so that as shown in FIG. 3, the axis of measurement of the object being measured 5 is positioned in alignment with the standard scale 2 and thereby the error due to the inclination is minimized. In such an instance, however, the problem has arisen that the instrument needs the overall length at least twice that of the object 5 with resultant oversizing of the instrument.
In contrast thereto, an apparatus has been proposed in which even when the axis of measurement of the object is positioned out of alignment with the standard scale, the error in Abbe's principle is not produced. This apparatus applies the so-called Eppenstein's principle and, for example, as shown in FIG. 4, includes a prism 6 and a lens 7 which are fixed to the base 1 not to move with respect to the stationary bed 3, and a lens 8 and a prism 9 which are disposed integral with the movable bed 4 so that an image 10' of an indicator 10 provided in the stationary bed 3 is projected onto the standard scale 2 through the prism 6, the lenses 7 and 8, and the prism 9, thereby causing the position of the movable bed 4 to be read out. If a distance f between the axis of measurement and the standard scale 2 is to be equal to the focal length of the lens 8 and as illustrated in FIG. 5 by way of example, the movable bed 4 is inclined at an angle .phi., an error equal nearly to f.phi. will be produced. Since, however, the lens 8 and the prism 9 are moved and inclined along with the movable bed 4 and a principal ray emanating from the indicator 10 is also inclined at the angle .phi., it follows that the ray is projected onto the standard scale at a position shifted by f.phi. in the direction that the error is offset and as a result, the image 10' of the indicator 10 is always projected at a proper position of the standard scale 2 without any error.
Any of the conventional examples stated above, however, has had the problems that the measurement can be made only in a one-dimensional direction, while in the case of the measurement of a two-dimensional object in a two-dimensional direction, it is required that the object is reset for twice-measurement with great trouble.